Prototyping of microfluidic devices in poly(dimethylsiloxane) using solid-object printing

Light-based 3D printing and post-treatments of moulds for PDMS soft lithography

Polydimethylsiloxane (PDMS) chips are still the workhorses of academic microfluidics. Their production requires the fabrication of moulds, commonly produced using clean-room technologies. Light-based 3D printing and in particular, vat photopolymerization, material jetting and two-photon polymerization are rising techniques for the fabrication of moulds for PDMS replication, thanks to their accessibility, fast prototyping time, and improving resolution. Here, we are first reviewing the possibility opened by 3D printing for soft lithography, with a focus on mould designs. Then, inhibition of PDMS curing by photosensitive resins will be discussed as the main technical hurdle of 3D printed moulds. Fortunately, mould post-treatments are efficient solutions to eliminate this curing inhibition, which we gathered in a large database of post-treatment protocols from the literature.

3D printing of micro-nano devices and their applications

In recent years, the utilization of 3D printing technology in micro and nano device manufacturing has garnered significant attention. Advancements in 3D printing have enabled achieving sub-micron level precision. Unlike conventional micro-machining techniques, 3D printing offers versatility in material selection, such as polymers. 3D printing technology has been gradually applied to the general field of microelectronic devices such as sensors, actuators and flexible electronics due to its adaptability and efficacy in microgeometric design and manufacturing processes. Furthermore, 3D printing technology has also been instrumental in the fabrication of microfluidic devices, both through direct and indirect processes. This paper provides an overview of the evolving landscape of 3D printing technology, delineating the essential materials and processes involved in fabricating microelectronic and microfluidic devices in recent times. Additionally, it synthesizes the diverse applications of these technologies across different domains.

Advantages of stereolithographic 3D printing in the fabrication of the Affiblot device for dot-blot assays

In stereolithographic (SLA) 3D printing, objects are constructed by exposing layers of photocurable resin to UV light. It is a highly user-friendly fabrication method that opens a possibility for technology sharing through CAD file online libraries. Here, we present a prototyping procedure of a microfluidics-enhanced dot-blot device (Affiblot) designed for simple and inexpensive screening of affinity molecule characteristics (antibodies, oligonucleotides, cell receptors, etc.). The incorporation of microfluidic features makes sample processing user-friendly, less time-consuming, and less laborious, all performed completely on-device, distinguishing it from other dot-blot devices. Initially, the Affiblot device was fabricated using CNC machining, which required significant investment in manual post-processing and resulted in low reproducibility. Utilization of SLA 3D printing reduced the amount of manual post-processing, which significantly streamlined the prototyping process. Moreover, it enabled the fabrication of previously impossible features, including internal fluidic channels. While 3D printing of sub-millimeter microchannels usually requires custom-built printers, we were able to fabricate microfluidic features on a readily available commercial printer. Open microchannels in the size range 200-300 μm could be fabricated with reliable repeatability and sealed with a replaceable foil. Economic aspects of device fabrication are also discussed.

A Versatile Photocrosslinkable Silicone Composite for 3D Printing Applications

Embedded printing has emerged as a valuable tool for fabricating complex structures and microfluidic devices. Currently, an ample of amount of research is going on to develop new materials to advance its capabilities and increase its potential applications. Here, we demonstrate a novel, transparent, printable, photocrosslinkable, and tuneable silicone composite that can be utilized as a support bath or an extrudable ink for embedded printing. Its properties can be tuned to achieve ideal rheological properties, such as optimal self-recovery and yield stress, for use in 3D printing. When used as a support bath, it facilitated the generation microfluidic devices with circular channels of diameter up to 30 μm. To demonstrate its utility, flow focusing microfluidic devices were fabricated for generation of Janus microrods, which can be easily modified for multitude of applications. When used as an extrudable ink, 3D printing of complex-shaped constructs were achieved with integrated electronics, which greatly extends its potential applications towards soft robotics. Further, its biocompatibility was tested with multiple cell types to validate its applicability for tissue engineering. Altogether, this material offers a myriad of potential applications (i.e., soft robotics, microfluidics, bioprinting) by providing a facile approach to develop complicated 3D structures and interconnected channels.

Versatile Microfluidics for Biofabrication Platforms Enabled by an Agile and Inexpensive Fabrication Pipeline

Microfluidics have transformed diagnosis and screening in regenerative medicine. Recently, they are showing much promise in biofabrication. However, their adoption is inhibited by costly and drawn-out lithographic processes thus limiting progress. Here, multi-material fibers with complex core-shell geometries with sizes matching those of human arteries and arterioles are fabricated employing versatile microfluidic devices produced using an agile and inexpensive manufacturing pipeline. The pipeline consists of material extrusion additive manufacturing with an innovative continuously varied extrusion (CONVEX) approach to produce microfluidics with complex seamless geometries including, novel variable-width zigzag (V-zigzag) mixers with channel widths ranging from 100-400 µm and hydrodynamic flow-focusing components. The microfluidic systems facilitated rapid mixing of fluids by decelerating the fluids at specific zones to allow for increased diffusion across the interfaces. Better mixing even at high flow rates (100-1000 µL min-1 ) whilst avoiding turbulence led to high cell cytocompatibility (>86%) even when 100 µm nozzles are used. The presented 3D-printed microfluidic system is versatile, simple and efficient, offering a great potential to significantly advance the microfluidic platform in regenerative medicine.

Rapid Micromolding of Sub-100 µm Microfluidic Channels Using an 8K Stereolithographic Resin 3D Printer

Engineering microfluidic devices relies on the ability to manufacture sub-100 micrometer fluidic channels. Conventional lithographic methods provide high resolution but require costly exposure tools and outsourcing of masks, which extends the turnaround time to several days. The desire to accelerate design/test cycles has motivated the rapid prototyping of microfluidic channels; however, many of these methods (e.g., laser cutters, craft cutters, fused deposition modeling) have feature sizes of several hundred microns or more. In this paper, we describe a 1-day process for fabricating sub-100 µm channels, leveraging a low-cost (USD 600) 8K digital light projection (DLP) 3D resin printer. The soft lithography process includes mold printing, post-treatment, and casting polydimethylsiloxane (PDMS) elastomer. The process can produce microchannels with 44 µm lateral resolution and 25 µm height, posts as small as 400 µm, aspect ratio up to 7, structures with varying z-height, integrated reservoirs for fluidic connections, and a built-in tray for casting. We discuss strategies to obtain reliable structures, prevent mold warpage, facilitate curing and removal of PDMS during molding, and recycle the solvents used in the process. To our knowledge, this is the first low-cost 3D printer that prints extruded structures that can mold sub-100 µm channels, providing a balance between resolution, turnaround time, and cost (~USD 5 for a 2 × 5 × 0.5 cm3 chip) that will be attractive for many microfluidics labs.

Advances in microfluidic chips based on islet hormone-sensing techniques

Diabetes mellitus is a global health problem resulting from islet dysfunction or insulin resistance. The mechanisms of islet dysfunction are still under investigation. Islet hormone secretion is the main function of islets, and serves an important role in the homeostasis of blood glucose. Elucidating the detailed mechanism of islet hormone secretome distortion can provide clues for the treatment of diabetes. Therefore, it is crucial to develop accurate, real-time, labor-saving, high-throughput, automated, and cost-effective techniques for the sensing of islet secretome. Microfluidic chips, an elegant platform that combines biology, engineering, computer science, and biomaterials, have attracted tremendous interest from scientists in the field of diabetes worldwide. These tiny devices are miniatures of traditional experimental systems with more advantages of time-saving, reagent-minimization, automation, high-throughput, and online detection. These features of microfluidic chips meet the demands of islet secretome analysis and a variety of chips have been designed in the past 20 years. In this review, we present a brief introduction of microfluidic chips, and three microfluidic chips-based islet hormone sensing techniques. We focus mainly on the theory of these techniques, and provide detailed examples based on these theories with the hope of providing some insights into the design of future chips or whole detection systems.

Label-Free Isolation of Exosomes Using Microfluidic Technologies

Exosomes are cell-derived structures packaged with lipids, proteins, and nucleic acids. They exist in diverse bodily fluids and are involved in physiological and pathological processes. Although their potential for clinical application as diagnostic and therapeutic tools has been revealed, a huge bottleneck impeding the development of applications in the rapidly burgeoning field of exosome research is an inability to efficiently isolate pure exosomes from other unwanted components present in bodily fluids. To date, several approaches have been proposed and investigated for exosome separation, with the leading candidate being microfluidic technology due to its relative simplicity, cost-effectiveness, precise and fast processing at the microscale, and amenability to automation. Notably, avoiding the need for exosome labeling represents a significant advance in terms of process simplicity, time, and cost as well as protecting the biological activities of exosomes. Despite the exciting progress in microfluidic strategies for exosome isolation and the countless benefits of label-free approaches for clinical applications, current microfluidic platforms for isolation of exosomes are still facing a series of problems and challenges that prevent their use for clinical sample processing. This review focuses on the recent microfluidic platforms developed for label-free isolation of exosomes including those based on sieving, deterministic lateral displacement, field flow, and pinched flow fractionation as well as viscoelastic, acoustic, inertial, electrical, and centrifugal forces. Further, we discuss advantages and disadvantages of these strategies with highlights of current challenges and outlook of label-free microfluidics toward the clinical utility of exosomes.

Pitting of malaria parasites in microfluidic devices mimicking spleen interendothelial slits

The spleen is a hematopoietic organ that participates in cellular and humoral immunity. It also serves as a quality control mechanism for removing senescent and/or poorly deformable red blood cells (RBCs) from circulation. Pitting is a specialized process by which the spleen extracts particles, including malaria parasites, from within circulating RBCs during their passage through the interendothelial slits (IES) in the splenic cords. To study this physiological function in vitro, we have developed two microfluidic devices modeling the IES, according to the hypothesis that at a certain range of mechanical stress on the RBC, regulated through both slit size and blood flow, would force it undergo the pitting process without affecting the cell integrity. To prove its functionality in replicating pitting of malaria parasites, we have performed a characterization of P. falciparum-infected RBCs (P.f.-RBCs) after their passage through the devices, determining hemolysis and the proportion of once-infected RBCs (O-iRBCs), defined by the presence of a parasite antigen and absence of DAPI staining of parasite DNA using a flow cytometry-based approach. The passage of P.f.-RBCs through the devices at the physiological flow rate did not affect cell integrity and resulted in an increase of the frequency of O-iRBCs. Both microfluidic device models were capable to replicate the pitting of P.f.-RBCs ex vivo by means of mechanical constraints without cellular involvement, shedding new insights on the role of the spleen in the pathophysiology of malaria.

Nylon Monofilament Mold Three-dimensional Microfluidic Chips for Size-exclusion Microchip Electrophoresis: Application to Specific Online Preconcentration of Proteins

We present a lithography-free procedure for fabricating intrinsically three-dimensional microchannels within PDMS elastomers using nylon monofilament molds. We embedded nylon monofilaments in an uncured PDMS composite to fabricate straight channels of desired length, for use as molds to form the microchannels. Next, we fabricated two layer devices consisting of dialysis membranes, which preconcentrate specific proteins in accordance with molecular weight, in between two layers of PDMS substrates with embedded microchannels. Because of the membrane isolation, analyte exchange between two fluidic layers can be precisely controlled by an applied voltage. More importantly, given that only small molecules pass through the dialysis membrane, the integrated membrane is suitable for molecular sieving or size exclusion for a concentrator prior to microchip electrophoresis. Researchers can use our microchip design for online purification and preconcentration of proteins in the presence of excess reagent immediately after fluorescent labeling. This method's technical advantage is that three-dimensional microstructures, such as microchannels that have a circular cross-section, are readily attainable and can be fabricated in a straightforward manner without using specialized equipment. Our method is a low-cost, environmentally sustainable procedure for fabricating microfluidic devices, and will render microfluidic processes more accessible and easy to implement.

Microfluidics for Drug Development: From Synthesis to Evaluation

Drug development is a long process whose main content includes drug synthesis, drug delivery, and drug evaluation. Compared with conventional drug development procedures, microfluidics has emerged as a revolutionary technology in that it offers a miniaturized and highly controllable environment for bio(chemical) reactions to take place. It is also compatible with analytical strategies to implement integrated and high-throughput screening and evaluations. In this review, we provide a comprehensive summary of the entire microfluidics-based drug development system, from drug synthesis to drug evaluation. The challenges in the current status and the prospects for future development are also discussed. We believe that this review will promote communications throughout diversified scientific and engineering communities that will continue contributing to this burgeoning field.

PDMS Curing Inhibition on 3D-Printed Molds: Why? Also, How to Avoid It?

Three-dimensional (3D)-printing techniques such as stereolithography (SLA) are currently gaining momentum for the production of miniaturized analytical devices and molds for soft lithography. However, most commercially available SLA resins inhibit polydimethylsiloxane (PDMS) curing, impeding reliable replication of the 3D-printed structures in this elastomeric material. Here, we report a systematic study, using 16 commercial resins, to identify a fast and straightforward treatment of 3D-printed structures and to support accurate PDMS replication using UV and/or thermal post-curing. In-depth analysis using Raman spectroscopy, nuclear magnetic resonance, and high-resolution mass spectrometry revealed that phosphine oxide-based photo-initiators, leaching out of the 3D-printed structures, are poisoning the Pt-based PDMS catalyst. Yet, upon UV and/or thermal treatments, photo-initiators were both eliminated and recombined into high molecular weight species that were sequestered in the molds.

Evaluation of 3D-printed molds for fabrication of non-planar microchannels

Replica obtained from micromolds patterned by simple photolithography has features with uniform heights, and attainable microchannels are thus quasi-two-dimensional. Recent progress in three-dimensional (3D) printing has enabled facile desktop fabrication of molds to replicate microchannels with varying heights. We investigated the replica obtained from four common techniques of 3D printing-fused deposition modeling, selective laser sintering, photo-polymer inkjet printing (PJ), and stereolithography (SL)-for the suitability to form microchannels in terms of the surface roughness inherent to the mechanism of 3D printing. There have been limited quantitative studies that focused on the surface roughness of a 3D-printed mold with different methods of 3D printing. We discussed that the surface roughness of the molds affected (1) transparency of the replica and (2) delamination pressure of poly(dimethylsiloxane) replica bonded to flat glass substrates. Thereafter, we quantified the accuracy of replication from 3D-printed molds by comparing the dimensions of the replicated parts to the designed dimensions and tested the ability to fabricate closely spaced microchannels. This study suggested that molds printed by PJ and SL printers were suitable for replica molding to fabricate microchannels with varying heights. The insight from this study shall be useful to fabricate 3D microchannels with controlled 3D patterns of flows guided by the geometry of the microchannels.

A simple and low-cost approach for irreversible bonding of polymethylmethacrylate and polydimethylsiloxane at room temperature for high-pressure hybrid microfluidics

Microfluidic devices have been used progressively in biomedical research due to the advantages they offer, such as relatively low-cost, rapid and precise processing, and an ability to support highly automated analyses. Polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA) are both biocompatible materials widely used in microfluidics due to their desirable characteristics. It is recognized that combining these two particular materials in a single microfluidic device would enable the development of an increasingly in-demand array of new applications, including those requiring high flow rates and elevated pressures. Whereas complicated and time-consuming efforts have been reported for bonding these two materials, the robust adhesion of PDMS and PMMA has not yet been accomplished, and remains a challenge. In this study, a new, simple, efficient, and low-cost method has been developed to mediate a strong bond between PMMA and PDMS layers at room temperature in less than 5 min using biocompatible adhesive tape and oxygen plasma treatment. The PDMS-PMMA bond was hydrolytically stable, and could tolerate a high influx of fluid without any leakage. This study addresses the limitations of existing approaches to bond these materials, and will enable the development of highly sought high-pressure and high-throughput biomedical applications.

A practical guide to rapid-prototyping of PDMS-based microfluidic devices: A tutorial

Micro total analytical systems (μTAS) are attractive to multiple fields that include chemistry, medicine and engineering due to their portability, low power usage, potential for automation, and low sample and reagent consumption, which in turn results in low waste generation. The development of fully-functional μTAS is an iterative process, based on the design, fabrication and testing of multiple prototype microdevices. Typically, microfabrication protocols require a week or more of highly-skilled personnel time in high-maintenance cleanroom facilities, which makes this iterative process cost-prohibitive in many locations worldwide. Rapid-prototyping tools, in conjunction with the use of polydimethylsiloxane (PDMS), enable rapid development of microfluidic structures at lower costs, circumventing these issues in conventional microfabrication techniques. Multiple rapid-prototyping methods to fabricate PDMS-based microfluidic devices have been demonstrated in literature since the advent of soft-lithography in 1998; each method has its unique advantages and drawbacks. Here, we present a tutorial discussing current rapid-prototyping techniques to fabricate PDMS-based microdevices, including soft-lithography, print-and-peel and scaffolding techniques, among other methods, specifically comparing resolution of the features, fabrication processes and associated costs for each technique. We also present thoughts and insights towards each step of the iterative microfabrication process, from design to testing, to improve the development of fully-functional PDMS-based microfluidic devices at faster rates and lower costs.

A 3D Printed Jet Mixer for Centrifugal Microfluidic Platforms

Homogeneous mixing of microscopic volume fluids at low Reynolds number is of great significance for a wide range of chemical, biological, and medical applications. An efficient jet mixer with arrays of micronozzles was designed and fabricated using additive manufacturing (three-dimensional (3D) printing) technology for applications in centrifugal microfluidic platforms. The contact surface of miscible liquids was enhanced significantly by impinging plumes from two opposite arrays of micronozzles to improve mixing performance. The mixing efficiency was evaluated and compared with the commonly used Y-shaped micromixer. Effective mixing in the jet mixer was achieved within a very short timescale (3s). This 3D printed jet mixer has great potential to be implemented in applications by being incorporated into multifarious 3D printing devices in microfluidic platforms.

Towards an Insulin Resistant Adipose Model on a Chip

Introduction

Adipose tissue and adipocytes are primary regulators of insulin sensitivity and energy homeostasis. Defects in insulin sensitivity of the adipocytes predispose the body to insulin resistance (IR) that could lead to diabetes. However, the mechanisms mediating adipocyte IR remain elusive, which emphasizes the need to develop experimental models that can validate the insulin signaling pathways and discover new mechanisms in the search for novel therapeutics. Currently in vitro adipose organ-chip devices show superior cell function over conventional cell culture. However, none of these models represent disease states. Only when these in vitro models can represent both healthy and disease states, they can be useful for developing therapeutics. Here, we establish an organ-on-chip model of insulin-resistant adipocytes, as well as characterization in terms of insulin signaling pathway and lipid metabolism.

Methods

We differentiated, maintained, and induced insulin resistance into primary adipocytes in a microfluidic organ-on-chip. We then characterized IR by looking at the insulin signaling pathway and lipid metabolism, and validated by studying a diabetic drug, rosiglitazone.

Results

We confirmed the presence of insulin resistance through reduction of Akt phosphorylation, Glut4 expression, Glut4 translocation and glucose uptake. We also confirmed defects of disrupted insulin signaling through reduction of lipid accumulation from fatty acid uptake and elevation of glycerol secretion. Testing with rosiglitazone showed a significant improvement in insulin sensitivity and fatty acid metabolism as suggested by previous reports.

Conclusions

The adipose-chip exhibited key characteristics of IR and can serve as model to study diabetes and facilitate discovery of novel therapeutics.

Scalable microfabrication of drug-loaded core-shell tablets from a single erodible polymer with adjustable release profiles

Oral tablets with tunable release profiles have emerged to enhance the effectiveness of therapies in different clinical conditions. Although the concept of tablets with adjustable release profiles has been studied before, the lack of a fast and scalable production technique has limited their widespread application. In this study, a scalable fabrication method was developed to manufacture controlled-release polyanhydride tablets. A new polymeric core-shell tablet design is also proposed, that in conjunction with a micro-fabrication procedure, allows for achieving tunable release profiles required in personalized medicine in small-size tablets. Utilizing a surface-erodible polymeric carrier in the fabrication of the new tablet design resulted in achieving adjustable release profiles and improvements in the drug-loading capacity of the delivery system which allows for delivering a flexible amount of therapeutics with desirable patterns to patients. The proposed fabrication techniques allow for scalable production of personalized tablets with the high resolution required in precision medicine and hence have a high potential for clinical translation.

In-situ time resolved spectrographic measurement using an additively manufactured metallic micro-fluidic analysis platform

Introduction

Microfluidic reactionware allows small volumes of reagents to be utilized for highly controlled flow chemistry applications. By integrating these microreactors with onboard analytical systems, the devices change from passive ones to active ones, increasing their functionality and usefulness. A pressing application for these active microreactors is the monitoring of reaction progress and intermediaries with respect to time, shedding light on important information about these real-time synthetic processes.

Objective

In this multi-disciplinary study the objective was to utilise advanced digital fabrication to research metallic, active microreactors with integrated fibre optics for reaction progress monitoring of solvent based liquids, incompatible with previously researched polymer devices, in combination with on-board Ultraviolet-visible spectroscopy for real-time reaction monitoring.

Method

A solid-state, metal-based additive manufactured system (Ultrasonic Additive Manufacturing) combined with focussed ion beam milling, that permitted the accurate embedment of delicate sensory elements directly at the point of need within aluminium layers, was researched as a method to create active, metallic, flow reactors with on-board sensing. This outcome was then used to characterise and correctly identify concentrations of UV-active water-soluble B-vitamin nicotinamide and fluorescein. A dilution series was formed from 0.01–1.75 mM; which was pumped through the research device and monitored using UV-vis spectroscopy.

Results

The results uniquely showed the in-situ ion milling of ultrasonically embedded optical fibres resulted in a metallic microfluidic reaction and monitoring device capable of measuring solvent solutions from 18 μM to 18 mM of nicotinamide and fluorescein, in real time. This level of accuracy highlights that the researched device and methods are capable of real-time spectrographic analysis of a range of chemical reactions outside of those possible with polymer devices.

Fabrication of Different Microchannels by Adjusting the Extrusion Parameters for Sacrificial Molds

Using the 3D printed mold-removal method to fabricate microchannel has become a promising alternative to the conventional soft lithography technique, due to the convenience in printing channel mold and the compatibility with PDMS material. Although having great potential, the use of single filament extruded by fused deposition modeling (FDM) as the sacrificial channel mold has not been elaborately studied. In this paper, we demonstrate the fabrication of microchannels with different structure and size by controllably extruding the sacrificial channel molds. The influences of the main processing parameters including working distance, extrusion amount and printing speed on the printed microchannels are systematically investigated. The results show that, the circular and low-aspect-ratio straight microchannels with different sizes can be fabricated by adjusting the extrusion amounts. The sinusoidal, 3D curved and cross-linked curved microchannels along straight path can be fabricated, either independently or in combination, by the combined control of the working distance, extrusion amount and printing speed. The complex microchannels with different structural features can also be printed along curved serpentine, rectangular serpentine, and spiral paths. This paper presents a simple and powerful method to fabricate the complex microchannels with different structure and size by just controlling the processing parameters for extruding channel molds.

The comparison between force volume and peakforce quantitative nanomechanical mode of atomic force microscope in detecting cell's mechanical properties

Atomic force microscope (AFM) has been widely used in the biological field owing to its high sensitivity (subnanonewton), high spatial resolution (nanometer), and adaptability to physiological environments. Nowadays, force volume (FV) and peakforce quantitative nanomechanical (QNM) are two distinct modes of AFM used in biomechanical research. However, numerous studies have revealed an extremely confusing phenomenon that FV mode has a significant difference with QNM in determining the mechanical properties of the same samples. In this article, for the case of human benign prostatic hyperplasia cells (BPH) and two cancerous prostate cells with different grades of malignancy (PC3 and DU145), the differences were compared between FV and QNM modes in detecting mechanical properties. The results show measured Young's modulus of the same cells in FV mode was much lower than that obtained by QNM mode. Combining experimental results with working principles of two modes, it is indicated that surface adhesion is highly suspected to be a critical factor resulting in the measurement difference between two modes. To further confirm this conjecture, various weight ratios of polydimethylsiloxane (PDMS) were assessed by two modes, respectively. The results show that the difference of Young's modulus measured by two modes increases with the surface adhesion of PDMS, confirming that adhesion is one of the significant elements that lead to the measurement difference between FV and QNM modes.

On the Impact of the Fabrication Method on the Performance of 3D Printed Mixers

A wide variety of 3D printing technologies have been used for the fabrication of lab-on-a-chip (LOC) devices in recent years. Despite the large number of studies having examined the use of 3D printing technologies in microfluidic devices, the effect of the fabrication method on their performance has received little attention. In this paper, a comparison is shown between unibody-LOC micro-mixers, a particular type of monolithic design for 3D printed LOCs, fabricated in polyjet, stereolithography (SLA) and fused deposition modelling (FDM or FFF) platforms, paying particular attention to the inherent limitations of each fabrication platform and how these affect the performance of the manufactured devices.

Fabrication of arbitrary three-dimensional suspended hollow microstructures in transparent fused silica glass

Fused silica glass is the preferred material for applications which require long-term chemical and mechanical stability as well as excellent optical properties. The manufacturing of complex hollow microstructures within transparent fused silica glass is of particular interest for, among others, the miniaturization of chemical synthesis towards more versatile, configurable and environmentally friendly flow-through chemistry as well as high-quality optical waveguides or capillaries. However, microstructuring of such complex three-dimensional structures in glass has proven evasive due to its high thermal and chemical stability as well as mechanical hardness. Here we present an approach for the generation of hollow microstructures in fused silica glass with high precision and freedom of three-dimensional designs. The process combines the concept of sacrificial template replication with a room-temperature molding process for fused silica glass. The fabricated glass chips are versatile tools for, among other, the advance of miniaturization in chemical synthesis on chip.

Fused silica glass has excellent optical properties, chemical and thermal stability and hardness, but its microstructuring for miniaturized applications has proven difficult. Here the authors demonstrate obtainment of precise arbitrary three dimensional hollow microstructures in fused silica glass by sacrificial template replication.

Low-budget 3D-printed equipment for continuous flow reactions

This article describes the development and manufacturing of lab equipment, which is needed for the use in flow chemistry. We developed a rack of four syringe pumps controlled by one Arduino computer, which can be manufactured with a commonly available 3D printer and readily available parts. Also, we printed various flow reactor cells, which are fully customizable for each individual reaction. With this equipment we performed some multistep glycosylation reactions, where multiple 3D-printed flow reactors were used in series.

Solid-State Nanopore Easy Chip Integration in a Cheap and Reusable Microfluidic Device for Ion Transport and Polymer Conformation Sensing

Solid-state nanopores have a huge potential in upcoming societal challenging applications in biotechnologies, environment, health, and energy. Nowadays, these sensors are often used within bulky fluidic devices that can cause cross-contaminations and risky nanopore chips manipulations, leading to a short experimental lifetime. We describe the easy, fast, and cheap innovative 3D-printer-helped protocol to manufacture a microfluidic device permitting the reversible integration of a silicon based chip containing a single nanopore. We show the relevance of the shape of the obtained channels thanks to finite elements simulations. We use this device to thoroughly investigate the ionic transport through the solid-state nanopore as a function of applied voltage, salt nature, and concentration. Furthermore, its reliability is proved through the characterization of a polymer-based model of protein-urea interactions on the nanometric scale thanks to a hairy nanopore.

A Review of Current Methods in Microfluidic Device Fabrication and Future Commercialization Prospects

Microfluidic devices currently play an important role in many biological, chemical, and engineering applications, and there are many ways to fabricate the necessary channel and feature dimensions. In this review, we provide an overview of microfabrication techniques that are relevant to both research and commercial use. A special emphasis on both the most practical and the recently developed methods for microfluidic device fabrication is applied, and it leads us to specifically address laminate, molding, 3D printing, and high resolution nanofabrication techniques. The methods are compared for their relative costs and benefits, with special attention paid to the commercialization prospects of the various technologies.

Microfluidic fabrication of microparticles for biomedical applications

Droplet microfluidics offers exquisite control over the flows of multiple fluids in microscale, enabling fabrication of advanced microparticles with precisely tunable structures and compositions in a high throughput manner. The combination of these remarkable features with proper materials and fabrication methods has enabled high efficiency, direct encapsulation of actives in microparticles whose features and functionalities can be well controlled. These microparticles have great potential in a wide range of bio-related applications including drug delivery, cell-laden matrices, biosensors and even as artificial cells. In this review, we briefly summarize the materials, fabrication methods, and microparticle structures produced with droplet microfluidics. We also provide a comprehensive overview of their recent uses in biomedical applications. Finally, we discuss the existing challenges and perspectives to promote the future development of these engineered microparticles.

Extrusion-Based 3D Printing of Microfluidic Devices for Chemical and Biomedical Applications: A Topical Review

One of the most widespread additive manufacturing (AM) technologies is fused deposition modelling (FDM), also known as fused filament fabrication (FFF) or extrusion-based AM. The main reasons for its success are low costs, very simple machine structure, and a wide variety of available materials. However, one of the main limitations of the process is its accuracy and finishing. In spite of this, FDM is finding more and more applications, including in the world of micro-components. In this world, one of the most interesting topics is represented by microfluidic reactors for chemical and biomedical applications. The present review focusses on this research topic from a process point of view, describing at first the platforms and materials and then deepening the most relevant applications.

Accessing microfluidics through feature-based design software for 3D printing

Additive manufacturing has been a cornerstone of the product development pipeline for decades, playing an essential role in the creation of both functional and cosmetic prototypes. In recent years, the prospects for distributed and open source manufacturing have grown tremendously. This growth has been enabled by an expanding library of printable materials, low-cost printers, and communities dedicated to platform development. The microfluidics community has embraced this opportunity to integrate 3D printing into the suite of manufacturing strategies used to create novel fluidic architectures. The rapid turnaround time and low cost to implement these strategies in the lab makes 3D printing an attractive alternative to conventional micro- and nanofabrication techniques. In this work, the production of multiple microfluidic architectures using a hybrid 3D printing-soft lithography approach is demonstrated and shown to enable rapid device fabrication with channel dimensions that take advantage of laminar flow characteristics. The fabrication process outlined here is underpinned by the implementation of custom design software with an integrated slicer program that replaces less intuitive computer aided design and slicer software tools. Devices are designed in the program by assembling parameterized microfluidic building blocks. The fabrication process and flow control within 3D printed devices were demonstrated with a gradient generator and two droplet generator designs. Precise control over the printing process allowed 3D microfluidics to be printed in a single step by extruding bridge structures to ‘jump-over’ channels in the same plane. This strategy was shown to integrate with conventional nanofabrication strategies to simplify the operation of a platform that incorporates both nanoscale features and 3D printed microfluidics.

Developing Microfluidic Sensing Devices Using 3D Printing

This short perspective assesses the present landscape for desktop 3D printing to design and fabricate sensors, in particular, those associated with microfluidics and multiplexing. Lots of advanced devices have already been reported, and this article briefly surveys interesting achievements. Microfluidics can be designed and optimized faster and more easily on low cost desktop 3D printers that with competing methods. Rapid prototyping leads directly to a final (marketable) product fabricated on the same 3D printer. While resolution is not as good as lithographic approaches, very often channel and feature resolution on the order of 100 μm obtainable with SLA 3D printers is perfectly suitable for the desired sensing device. Two examples from our team's research are used to illustrate how using a 3D printer along with simple automation can reduce a complex microfluidic sensing procedure to a much simpler automated one. Future possibilities for sensor technology are discussed.

Towards microbioprocess control: an inexpensive 3D printed microbioreactor with integrated online real-time glucose monitoring

A 3D printed micro-bioreactor and microfluidic chip with integrated screen printed glucose biosensor for online monitoring of glucose to aid micro-bioprocess control.

Suspended Liquid Subtractive Lithography: One-step generation of 3D channel geometries in viscous curable polymer matrices

The miniaturization of synthesis, analysis and screening experiments is an important step towards more environmentally friendly chemistry, statistically significant biology and fast and cost-effective medicinal assays. The facile generation of arbitrary 3D channel structures in polymers is pivotal to these techniques. Here we present a method for printing microchannels directly into viscous curable polymer matrices by injecting a surfactant into the uncured material via a steel capillary attached to a 3D printer. We demonstrate this technique using polydimethylsiloxane (PDMS) one of the most widely used polymers for the fabrication of, e. g. microfluidic chips. We show that this technique which we term Suspended Liquid Subtractive Lithography (SLSL) is well suited for printing actuators, T-junctions and complex three dimensional structures. The formation of truly arbitrary channels in 3D could revolutionize the fabrication of miniaturized chips and will find broad application in biology, chemistry and medicine.

Comparing Microfluidic Performance of Three-Dimensional (3D) Printing Platforms

Three-dimensional (3D) printing has emerged as a potential revolutionary technology for the fabrication of microfluidic devices. A direct experimental comparison of the three 3D printing technologies dominating microfluidics was conducted using a Y-junction microfluidic device, the design of which was optimized for each printer: fused deposition molding (FDM), Polyjet, and digital light processing stereolithography (DLP-SLA). Printer performance was evaluated in terms of feature size, accuracy, and suitability for mass manufacturing; laminar flow was studied to assess their suitability for microfluidics. FDM was suitable for microfabrication with minimum features of 321 ± 5 μm, and rough surfaces of 10.97 μm. Microfluidic devices >500 μm, rapid mixing (71% ± 12% after 5 mm, 100 μL/min) was observed, indicating a strength in fabricating micromixers. Polyjet fabricated channels with a minimum size of 205 ± 13 μm, and a surface roughness of 0.99 μm. Compared with FDM, mixing decreased (27% ± 10%), but Polyjet printing is more suited for microfluidic applications where flow splitting is not required, such as cell culture or droplet generators. DLP-SLA fabricated a minimum channel size of 154 ± 10 μm, and 94 ± 7 μm for positive structures such as soft lithography templates, with a roughness of 0.35 μm. These results, in addition to low mixing (8% ± 1%), showed suitability for microfabrication, and microfluidic applications requiring precise control of flow. Through further discussion of the capabilities (and limitations) of these printers, we intend to provide guidance toward the selection of the 3D printing technology most suitable for specific microfluidic applications.

An easily fabricated three-dimensional threaded lemniscate-shaped micromixer for a wide range of flow rates

Mixing fluid samples or reactants is a paramount function in the fields of micro total analysis system (μTAS) and microchemical processing. However, rapid and efficient fluid mixing is difficult to achieve inside microchannels because of the difficulty of diffusive mass transfer in the laminar regime of the typical microfluidic flows. It has been well recorded that the mixing efficiency can be boosted by migrating from two-dimensional (2D) to three-dimensional (3D) geometries. Although several 3D chaotic mixers have been designed, most of them offer a high mixing efficiency only in a very limited range of Reynolds numbers (Re). In this work, we developed a 3D fine-threaded lemniscate-shaped micromixer whose maximum numerical and empirical efficiency is around 97% and 93%, respectively, and maintains its high performance (i.e., >90%) over a wide range of 1 < Re < 1000 which meets the requirements of both the μTAS and microchemical process applications. The 3D micromixer was designed based on two distinct mixing strategies, namely, the inducing of chaotic advection by the presence of Dean flow and diffusive mixing through thread-like grooves around the curved body of the mixers. First, a set of numerical simulations was performed to study the physics of the flow and to determine the essential geometrical parameters of the mixers. Second, a simple and cost-effective method was exploited to fabricate the convoluted structure of the micromixers through the removal of a 3D-printed wax structure from a block of cured polydimethylsiloxane. Finally, the fabricated mixers with different threads were tested using a fluorescent microscope demonstrating a good agreement with the results of the numerical simulation. We envisage that the strategy used in this work would expand the scope of the micromixer technology by broadening the range of efficient working flow rate and providing an easy way to the fabrication of 3D convoluted microstructures.

Fully inkjet-printed microfluidics: a solution to low-cost rapid three-dimensional microfluidics fabrication with numerous electrical and sensing applications

As the needs for low-cost rapidly-produced microfluidics are growing with the trend of Lab-on-a-Chip and distributed healthcare, the fully inkjet-printing of microfluidics can be a solution to it with numerous potential electrical and sensing applications. Inkjet-printing is an additive manufacturing technique featuring no material waste and a low equipment cost. Moreover, similar to other additive manufacturing techniques, inkjet-printing is easy to learn and has a high fabrication speed, while it offers generally a great planar resolution down to below 20 µm and enables flexible designs due to its inherent thin film deposition capabilities. Due to the thin film feature, the printed objects also usually obtain a high vertical resolution (such as 4.6 µm). This paper introduces a low-cost rapid three-dimensional fabrication process of microfluidics, that relies entirely on an inkjet-printer based single platform and can be implemented directly on top of virtually any substrates.

A reconfigurable stick-n-play modular microfluidic system using magnetic interconnects

A reconfigurable "stick-n-play" modular microfluidic system that can be assembled, disassembled, reconfigured and assembled again for building different integrated microfluidic systems is presented. Magnetic interconnects, comprising ring magnets and sealing gaskets, are integrated into each microfluidic module's inlet(s) and outlet(s) for both module-to-module and world-to-chip fluidic interconnects. The magnetic interconnects reversibly "stick" each individual microfluidic module together and provide leak-free fluidic communication between connected microfluidic modules in order to form a larger integrated microfluidic system. Because of the magnetic interconnects, connected microfluidic modules can be easily disconnected, reconfigured and connected again to form a different integrated microfluidic system. Using a fused deposition modeling (FDM)/fused filament fabrication (FFF)-based 3D printer, a reconfigurable stick-n-play modular microfluidic system, comprising a serpentine channel base platform and various microfluidic modules as well as inlet/outlet modules for world-to-chip fluidic interconnects, was first 3D printed. Magnetic interconnects were then integrated into each 3D printed module. Finally, the stick-n-play modular microfluidic system was used to demonstrate its reconfigurability to build various integrated microfluidic systems by simply and reversibly sticking various modules together. Based on the magnetic interconnects, customized multi-dimensional stick-n-play modular microfluidic systems can be easily designed and built providing a convenient platform for designing large scale microfluidic systems.

3D-printed Microfluidic Devices: Fabrication, Advantages and Limitations-a Mini Review

A mini-review with 79 references. In this review, the most recent trends in 3D-printed microfluidic devices are discussed. In addition, a focus is given to the fabrication aspects of these devices, with the supplemental information containing detailed instructions for designing a variety of structures including: a microfluidic channel, threads to accommodate commercial fluidic fittings, a flow splitter; a well plate, a mold for PDMS channel casting; and how to combine multiple designs into a single device. The advantages and limitations of 3D-printed microfluidic devices are thoroughly discussed, as are some future directions for the field.

3D-printed bioanalytical devices

While 3D printing technologies first appeared in the 1980s, prohibitive costs, limited materials, and the relatively small number of commercially available printers confined applications mainly to prototyping for manufacturing purposes. As technologies, printer cost, materials, and accessibility continue to improve, 3D printing has found widespread implementation in research and development in many disciplines due to ease-of-use and relatively fast design-to-object workflow. Several 3D printing techniques have been used to prepare devices such as milli- and microfluidic flow cells for analyses of cells and biomolecules as well as interfaces that enable bioanalytical measurements using cellphones. This review focuses on preparation and applications of 3D-printed bioanalytical devices.

3D printed microfluidic devices: enablers and barriers

3D printing has the potential to significantly change the field of microfluidics. The ability to fabricate a complete microfluidic device in a single step from a computer model has obvious attractions, but it is the ability to create truly three dimensional structures that will provide new microfluidic capability that is challenging, if not impossible to make with existing approaches. This critical review covers the current state of 3D printing for microfluidics, focusing on the four most frequently used printing approaches: inkjet (i3DP), stereolithography (SLA), two photon polymerisation (2PP) and extrusion printing (focusing on fused deposition modeling). It discusses current achievements and limitations, and opportunities for advancement to reach 3D printing's full potential.

Bio-functionalized silk hydrogel microfluidic systems

Bio-functionalized microfluidic systems were developed based on a silk protein hydrogel elastomeric materials. A facile multilayer fabrication method using gelatin sacrificial molding and layer-by-layer assembly was implemented to construct interconnected, three dimensional (3D) microchannel networks in silk hydrogels at 100 μm minimum feature resolution. Mechanically activated valves were implemented to demonstrate pneumatic control of microflow. The silk hydrogel microfluidics exhibit controllable mechanical properties, long-term stability in various environmental conditions, tunable in vitro and in vivo degradability in addition to optical transparency, providing unique features for cell/tissue-related applications than conventional polydimethylsiloxane (PDMS) and existing hydrogel-based microfluidic options. As demonstrated in the work here, the all aqueous-based fabrication process at ambient conditions enabled the incorporation of active biological substances in the bulk phase of these new silk microfluidic systems during device fabrication, including enzymes and living cells, which are able to interact with the fluid flow in the microchannels. These silk hydrogel-based microfluidic systems offer new opportunities in engineering active diagnostic devices, tissues and organs that could be integrated in vivo, and for on-chip cell sensing systems.

3D-printed supercapacitor-powered electrochemiluminescent protein immunoarray

Herein we report a low cost, sensitive, supercapacitor-powered electrochemiluminescent (ECL) protein immunoarray fabricated by an inexpensive 3-dimensional (3D) printer. The immunosensor detects three cancer biomarker proteins in serum within 35 min. The 3D-printed device employs hand screen printed carbon sensors with gravity flow for sample/reagent delivery and washing. Prostate cancer biomarker proteins, prostate specific antigen (PSA), prostate specific membrane antigen (PSMA) and platelet factor-4 (PF-4) in serum were captured on the antibody-coated carbon sensors followed by delivery of detection-antibody-coated Ru(bpy)3(2+) (RuBPY)-doped silica nanoparticles in a sandwich immunoassay. ECL light was initiated from RuBPY in the silica nanoparticles by electrochemical oxidation with tripropylamine (TPrA) co-reactant using supercapacitor power and ECL was captured with a CCD camera. The supercapacitor was rapidly photo-recharged between assays using an inexpensive solar cell. Detection limits were 300-500f gmL(-1) for the 3 proteins in undiluted calf serum. Assays of 6 prostate cancer patient serum samples gave good correlation with conventional single protein ELISAs. This technology could provide sensitive onsite cancer diagnostic tests in resource-limited settings with the need for only moderate-level training.

Hybrid PDMS/glass microfluidics for high resolution imaging and application to sub-wavelength particle trapping

We demonstrate the fabrication of a hybrid PDMS/glass microfluidic layer that can be placed on top of non-transparent samples and allows high-resolution optical microscopy through it. The layer mimics a glass coverslip to limit optical aberrations and can be applied on the sample without the use of permanent bonding. The bonding strength can withstand to hold up to 7 bars of injected pressure in the channel without leaking or breaking. We show that this process is compatible with multilayer soft lithography for the implementation of flexible valves. The benefits of this application is illustrated by optically trapping sub-wavelength particles and manipulate them around photonic nano-structures. Among others, we achieve close to diffraction limited imaging through the microfluidic assembly, full control on the flow with no dynamical deformations of the membrane and a 20-fold improvement on the stiffness of the trap at equivalent trapping power.

Facile and cost-effective production of microscale PDMS architectures using a combined micromilling-replica moulding (μMi-REM) technique

We describe a cost-effective and simple method to fabricate PDMS-based microfluidic devices by combining micromilling with replica moulding technology. It relies on the following steps: (i) microchannels are milled in a block of acrylic; (ii) low-cost epoxy adhesive resin is poured over the milled acrylic block and allowed to cure; (iii) the solidified resin layer is peeled off the acrylic block and used as a mould for transferring the microchannel architecture onto a PDMS layer; finally (iv) the PDMS layer is plasma bonded to a glass surface. With this method, microscale architectures can be fabricated without the need for advanced technological equipment or laborious and time-consuming intermediate procedures. In this manuscript, we describe and validate the microfabrication procedure, and we illustrate its applicability to emulsion and microbubble production.